Recombinant proteins are an emerging class of therapeutic agents. Such recombinant therapeutics have engendered advances in protein formulation and chemical modification. Such modifications can protect therapeutic proteins, primarily by blocking their exposure to proteolytic enzymes. Protein modifications may also increase the therapeutic protein's stability, circulation time, and biological activity. A review article describing protein modification and fusion proteins is Francis (1992), Focus on Growth Factors 3:4-10 (Mediscript, London), which is hereby incorporated by reference.
One useful modification is combination with the “Fc” domain of an antibody. Antibodies comprise two functionally independent parts, a variable domain known as “Fab”, which binds antigen, and another domain known as “Fc”, which links to such effector functions as complement activation and attack by phagocytic cells. An Fc has a long serum half-life, whereas an Fab is short-lived. Capon et al. (1989), Nature 337:525-31. When constructed together with a therapeutic protein, an Fc domain can provide longer half-life or incorporate such functions as Fc receptor binding, protein A binding, complement fixation and perhaps even placental transfer. Id. Table 1 summarizes use of Fc fusions known in the art.
TABLE 1Fc fusion with therapeutic proteinsFusionTherapeuticForm of FcpartnerimplicationsReferenceIgG1N-terminus ofHodgkin's disease;U.S. Pat. No.CD30-Lanaplastic lymphoma; T-5,480,981cell leukemiaMurine Fcγ2aIL-10anti-inflammatory;Zheng et al. (1995), J.transplant rejectionImmunol. 154: 5590-600IgG1TNF receptorseptic shockFisher et al. (1996), N.Engl. J. Med. 334: 1697-1702; Van Zee, K. et al.(1996), J. Immunol. 156:2221-30IgG, IgA,TNF receptorinflammation, autoimmuneU.S. Pat. No. 5,808,029,IgM, or IgEdisordersissued Sep. 15,(excluding1998the firstdomain)IgG1CD4 receptorAIDSCapon et al.(1989),Nature 337: 525-31IgG1,N-terminusanti-cancer, antiviralHarvill et al. (1995),IgG3of IL-2Immunotech. 1: 95-105IgG1C-terminus ofosteoarthritis;WO 97/23614, publishedOPGbone densityJul. 3, 1997IgG1N-terminus ofanti-obesityPCT/US 97/23183, filedleptinDec. 11, 1997Human IgCTLA-4autoimmune disordersLinsley (1991), J. Exp.Cγ1Med. 174: 561-9
Despite their advantages, use of Fc fusion molecules may be limited by misfolding upon expression in a desired cell line. Such misfolded Fc fusion molecules may generate an immune response in vivo or may cause aggregation or stability problems in production.